Cameras are listed differently than lenses and films, and cannot be added/edited in the same way. As one uploads a photo to the gallery, the camera manufacturer name and camera model are pulled from the EXIF info automatically. In the absence of EXIF, one can manually enter this in the Photo Detail Editor fields.
This gets messy, as the same model of camera is entered in different inconsistent ways by different (or at different times by even the same!) members.
In the interests of consistency, a member can populate his own camera list under the "My Menu" menu in the Gallery... see "Cameras" on that menu. At left is the full and inconsistent list compiled from everyone's entries. From this list, pick out the entry that you feel best describes your camera and click the checkbox. At the bottom is a button to save the changes, and the clicked camera model will then appear in your own personal camera list at right.
From here, when you post a new photo that doesn't have EXIF, then you can pick from a pop-up menu in the Photo Detail Editor reflecting your own camera list. I hope I've got this right, as it's been quite a while since I manually chose from that list...